It is often desirable to measure the thickness (or other properties) of a film or film stack during semiconductor processing. Generally, optical metrology techniques, such as reflectometry and ellipsometry, can be used to accurately measure a thin film or film stack. However, in order to accurately measure a film using optical metrology, a good understanding of the material under the film or films of interest is required.
In one conventional metrology technique, a single measurement of the sample is made. The film of interest and the underlying structure are then modeled mathematically. This technique is particularly useful when the underlying structure is a simple structure, such as a uniform film. Unfortunately, when the underlying structure is complicated, such as a two dimensional repeating pattern, analytically modeling the underlying structure can be prohibitively difficult to use this technique.
In another conventional metrology technique, the structure underlying the film of interest is measured prior to deposition of the film of interest. For example, the spectra from the underlying structure is measured, the film is then deposited, and the entire stack is then measured. The measured spectra from the underlying structure can then be subtracted from the spectra from the entire stack. This technique is particularly advantageous where the underlying structure is complicated. However, pre-measuring the underlying structure is undesirable, as it requires two separate measurements of the same location, which is time consuming and often requires removal of the substrate from the deposition tool.
Where there is a complicated structure underlying the film to be measured, conventionally, a separate measurement pad with no underlying structure (or a drastically simplified underlying structure) is used. The measurement pad is located, e.g., in a scribe line or other location on the wafer. While the use of a measurement pad permits easy measurement of the film on the measurement pad, the resulting measurement may not accurately describe the actual device, i.e., the film over the complicated structure. Thus, there is a desire to accurately measure the actual device itself, rather than a measurement pad.
Accordingly, what is needed is an improved optical metrology process that accurately measures one or more films that overlie a complicated structure without requiring two separate measurements.